Method and apparatus for drying and incineration of sewage sludge

ABSTRACT

A system for drying sludge includes a shaftless spiral feed screw for moving sludge through a drying chamber. An high energy inductor is located at a output of the drying chamber for drawing hot gases through the chamber to dry the sludge as it advances from the input end to the output end of the chamber. The high energy inductor also aspirates the dried sludge from the chamber. The drying system can be used in conjunction with a waste-to-energy furnace for incineration of sludge and municipal waste. In such an arrangement, the dried sludge can be aspirated from the drying chamber directly into a combustion zone of the furnace. Hot gases from the furnace can be used in drying the sludge.

RELATED APPLICATIONS

This application is a divisional application and claims priority to U.S.patent application Ser. No. 09/745,009, filed Dec. 20, 2000 U.S. Pat.No. 6,412,428.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus for disposal ofsewage sludge. In particular, it relates to a method and apparatus fordrying and combustion of sludge in connection with refuse incineration.

2. Discussion of the Related Art

Various methods have been developed over the years to dispose of sewagesludge, the discharge from waste water purification. Such methodsinclude landfilling, composting, wet spreading and drying forfertilizer. However, these methods are proving to be controversial andunacceptable in today's environment. Also, landfills have becomeincreasingly expensive as landfills are closed and transportation costshave increased to new locations. Landspreading is not sufficientlysanitary, resulting in leaching of some sludge byproducts into freshwater supply systems. Although sewage sludge is rich in nutrients, itsheavy metals, pathogens and other chemical contents have raisedquestions about its acceptability for spreading on farmland, in eitherthe wet or dry forms. Also, the drying of sewage sludge for fertilizerrequires significant energy, which has become more expensive.

More recently, incineration has developed as a way to dispose of sewagesludge and to reduce reliance on other fuel sources. Since sewage sludgehas a high water content, as much as 95%, dewatering and dryingprocesses are required prior to incineration. Typically, dewatering iscarried out with filter presses, belt filter presses or centrifuges.Drying uses cylindrical rotary kilns, kilns with overlying beds,fluidized bed furnaces, grinding driers or indirect driers of differentconstructions. A representative apparatus for incineration of sewagesludge is disclosed in U.S. Pat. No. 3,954,069. Another such apparatusis disclosed in U.S. Pat. No. 4,311,103. The steps of dewatering anddrying require significant energy and transport of the sewage sludge,with different water contents, to various locations for these processesto occur. These systems also require a complicated infrastructure withmultiple pieces of interconnected processing equipment.

In order to address disposal of both sewage sludge and household ormunicipal solid waste, different waste combustion systems have beendeveloped. Initial systems attempted to mix the sewage sludge andmunicipal solid waste together prior to being fed to the waste to energyboiler. However, in such a system, the overall water content of thesludge and waste results in poor combustion and slower feed rates. Toovercome this problem, one system, disclosed in U.S. Pat. No. 4,753,181,fed the sewage sludge separately from the municipal waste. In thissystem, the sewage sludge is dispersed in small droplets over the flamein front of the combustion zone. The droplets are dried and incineratedby the hot air in the furnace. However, since the sludge is still in aliquid state, the flow rate must be controlled to keep the moisturecontent in the furnace within acceptable levels.

U.S. Pat. No. 5,410,973 discloses a system which uses the flue gasesfrom refuse combustion and sewage sludge incineration in processing theother type of waste. The sewage sludge is incinerated in a rotarytubular kiln. Flue gases from the refuse furnace are drawn in theopposite direction of sludge flow for drying and incinerating the sludgein a single process stage. The flue gases from the kiln are fed backinto the refuse furnace for afterburning. This system, however, requirestwo distinct furnaces, one for sludge and one for refuse. The materialscannot be combined. Additionally, when the flue gasses are transferredbetween the furnaces, they must be kept hot enough so as not toadversely affect the incineration process.

U.S. Pat. No. 5,630,366 discloses another system in which the sewagesludge is dried before it passes to the incinerator. The sludge isdehydrated by centrifuging-drying using the heat of the fumes from therefuse furnace. The dried sludge is then combined with the municipalwaste to be incinerated. This system makes use of the hot gases from thefurnace in drying the sludge, but requires transportation of sludge invarious forms to different locations. Furthermore, the gases must bedirected to the drying chamber for the sludge.

Therefore, a need exists for a simple system for processing sewagesludge for incineration. A need exists for a system which can incineratesewage sludge in conjunction with municipal waste. A need exists for asystem which can dry sewage sludge before combustion without significantenergy uses and without requiring transport to multiple processingareas.

SUMMARY OF THE INVENTION

The present invention overcomes deficiencies of prior art systems byproviding a single processing stage for transporting and drying thesewage sludge, the dried sludge can be directly injected into a typicalmunicipal waste to energy plant incorporating the invention. The presentinvention includes a drying system having a spiral feed screw without acentral shaft. Heated air, which could be from a furnace, is directedthrough the center of the drying system for drying the sewage sludge. Ahigh energy inductor at the end of the feed system provides the draw forthe heated air. The high energy inductor also expels aspirated, driedsludge. When incorporated into a waste to energy plant, the dryingsystem can feed the dried sludge directly into the combustion zone ofthe furnace, resulting in combustion of the sludge. The municipal wasteto energy plant can be operated in an ordinary manner without the needfor additional energy or processing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a sewage sludge drying systemaccording to an embodiment of the present invention.

FIG. 2 is a cross sectional view of a typical municipal waste to energyboiler incorporating an embodiment of the present invention.

FIG. 3 is a partial cross sectional view of a typical municipal waste toenergy boiler incorporating an embodiment of the present invention.

FIG. 4 is a top view of the feed system shown in FIG. 3 along the lineIV—IV.

FIG. 5 is a partial cross sectional view of a typical municipal waste toenergy boiler incorporating a second embodiment of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The drying system of the present invention is illustrated in FIG. 1. Thedrying system 100 receives sewage sludge at one end 101 and dischargesdried sludge at the opposite end 102. The drying system 100 includes ascrew feeder 110 having a housing 111 and a shaftless screw 112 in thehousing 111. The housing 111 and shaftless screw 112 can be of variousmaterials, including carbon steel, stainless steel or other hybridalloys. The material used will depend upon the temperatures and otheroperating conditions of the drying system. The shaftless screw 112 canbe of any type, but a suitable shaftless screw is manufactured by Spiracof Sweden. Alternatively, a ribbon type screw conveyor could be usedrather than a shaftless screw. Of course, a shafted or multiple screwconveyor could also be used as long as air can flow through theconveyor. The shaftless screw is rotated by a drive motor 103 which canbe externally located at the feed end of the screw feeder 110.

At the discharge end 102 of the feed system is a high capacity blower120 with a venturi flow passage. The venturi flow passage causes thehigh capacity blower to move substantial amounts of flue gas with asmall air feed source. The high capacity blower 120 or inductor may varyin size and material depending upon the conditions of the sludge and theoperating temperatures. According to an embodiment of the presentinvention, as illustrated in FIG. 1, the gas feed source is provided bya hot air supply 121 piped 122 to the entrance of the high capacityblower 120. Fans, blowers or other devices (as shown) can be used toforce the hot gas into the high energy inductor for the gas feed source.Compressed air or steam may be used as a feed source for the highcapacity blower 120. An alternative thermal 124 heat source can be usedto heat the air for the hot air supply. The additional air moved by thehigh capacity blower also comes from the hot air. As illustrated in FIG.1, the hot gas can be drawn through ducts 130 along the screw feeder110. The hot gas enters through an inlet 131 at the feed end 101. Inthis manner, the housing 111 of the screw feeder 110 is heated and thesludge is heated through the surface and through heated gas passingthrough the feed system.

The shaftless screw 112 is driven using an external motor 103 to movethe sludge toward the high capacity blower 120. As the sludge is movedthrough the feed system 100, the hot furnace gas passing through thesystem dries the sludge. The moisture content of the sludge at thedischarge end 102 can be controlled by the speed of rotation of theshaftless screw 112 relative to the gas being moved through the feedsystem 100 and in relation to the rate at which sludge is added to thefeed system. The sludge input rate can be controlled using variablespeed drives or a pneumatic control valve system. When the sewage sludgereaches the discharge end of the feed system, it is sufficiently dry, tobe aspirated through the high capacity blower. The high volumes of gaspassing through the drying system causes aspiration and distribution ofthe sludge at the discharge end 102.

FIG. 2 illustrates a typical municipal waste to energy boiler 1, whichincorporates the present invention. The refuse is passed from a feedhopper 10, down a chute 11 to the furnace. A ram feeder 12 pushes therefuse onto a grate 1 of the furnace. The refuse passes along the grate13, where it is combusted. Air for combustion 14 is fed from below thegrate. Above the grate is an overfire combustion section 20. Thissection includes air inlets 21, 22 and auxiliary burners (not shown). Inaccordance with typical designs, the exhaust gases are used to producesteam and are processed to reduce emissions with state of the art airpollution control equipment. Ash passes through the grate and iscollected at the lower end of the furnace 15.

An embodiment of the present invention includes a feed system 100disposed within the furnace. The drying system is positioned on thefurnace so as to utilize the hot gases from the furnace for drying andto allow incineration of the dried sludge. In the embodiment of FIG. 2,the drying system 100 is located within the furnace 1. The heat source121 of the drying system can be eliminated in favor of hot gases fromthe furnace. The hot air ducts 130 can remain open at the discharge end102 of the drying system to draw on hot gases for the drying process. Asource of compressed air or steam is used as a feed source for the highcapacity blower 120.

During operation, sewage sludge, having up to 95% water content, ispumped 122 from the tipping floor or other supply source to the feed endof the feeding system 100. Although a positive displacement pump 122 isillustrated in FIG. 2, other types of conveyors may be used to move thesludge to the feeding system. Such conveyors could include phenumaticdense phase, hydraulic sludge pump, or drag, belt or screw conveyors.Due to the placement of the drying system in the furnace, the aspiratedsludge is discharged into the combustion zone of the furnace, preferablybelow the OFA zone, zone 20. In addition, multiple inductors or othertypes (different sizes) of inductors, in combination with or replacementfor the high capacity blower 120, can be used. Such inductors couldinclude different types of fans. FIG. 4 illustrates the placement of thefeed system 100 relative to the grate 13 of the furnace. Of course, thedrying system could be located anywhere such that the aspirated sludgeis feed into the furnace for combustion. Furthermore, the presentinvention could be used with different types of furnaces or boilers,such as solid fuel boilers with traveling or reciprocating grates,hydrogrates, and air or steam swept grates. The present invention couldalso be used on different fluid bed units.

FIG. 5 illustrates a second embodiment of the present invention. In thesecond embodiment, the feed system 100 is located externally from thefurnace 1. Ducts 130 can be used to draw heated air from the furnace fordrying the sludge. The dried sludge is still discharged into acombustion zone of the furnace.

Of course, other designs could be used for creating the drying system.And the drying systems could be used with different types of waste. Forexample, rather than municipal sewage sludge, the drying system of thepresent invention could be used with paper mill products such as pulpand paper sludges, wood chips, saw dust, and bark. The present inventionmay also be used to dry and incinerate animal wastes, including hog,horse, cow and poultry manure. The present invention may also be usedfor drying wastes without incineration. The drying system could bedirected into a storage or transportation bin so that the dried materialis aspirated into the bin for transport or other use.

Furthermore, those of ordinary skill in the art will recognize thatadaptations and modifications can be made to the embodiments withoutdeparting from the essential characteristics of the present invention.The scope of the invention is not limited by the embodiments disclosedand includes adaptations and modifications.

I claim:
 1. A method of drying waste having a high water content priorto incineration in a furnace, the method comprising the steps of:transporting the waste through a housing having an input end and anoutput end, the output end facing towards the furnace; flowing hot gasesthrough the housing from the input end to the output end, such that thehot gases contact and dry the waste; and discharging the hot gases fromthe output end of the housing at a velocity such that dried waste isexpelled from the housing into the furnace; and withdrawing hot exhaustgases from the furnace to the input end of the housing to flow throughthe housing.
 2. The method of drying waste according to claim 1, furthercomprising the step of flowing the hot exhaust gases from the furnacealong an exterior of the housing from the output end to the input end toheat the housing.
 3. An waste incineration system comprising: a furnacefor incinerating waste products having a combustion area; and a dryingsystem for drying waste having a high water content prior to entry intothe furnace, the drying system including: a housing formed as a tubehaving an input end and an output end, the output end opening towardsthe combustion area of the furnace; a transport system within thehousing for moving waste from the input end to the output end; a supplyof hot gases connected to an interior of the housing at the input end ofthe housing; and an air movement system connected to the interior of thehousing at the output end of the housing, the air movement systemexhausting gases from the interior of the housing to draw hot gases fromthe supply of hot gases through the housing such that the hot gasesmoving through the interior of the housing contact and dry the waste,and the air movement system further discharging the waste from theoutput end with the exhausting gases into the combustion area of thefurnace.
 4. The waste incineration system according to claim 3, whereinthe transport system includes: a screw mechanism disposed in thehousing; and a motor located externally from the housing for rotatingthe screw mechanism.
 5. The waste incineration system according to claim3, wherein the air movement system includes: a high capacity blowerattached to the output end of the housing; and an air feed sourceconnected between the output end of the housing and an input end of thehigh capacity, the air feed source providing an air flow to the highcapacity blower to drive the high capacity blower.
 6. The wasteincineration system according to claim 3, wherein the supply of hotgases includes: a heat source for heating air; and ducts for providingair heated by the heat source to the input end of the housing.
 7. Thewaste incineration system according to claim 3, further comprising atleast one duct located on an external surface of the housing forproviding hot gases from a source of hot gases located away from theinput end of the housing to the supply of hot gases at the input end ofthe housing; the at least one duct being positioned such that hot gasespassing through the duct heat a surface of the housing.
 8. The wasteincineration system according to claim 3, wherein the drying system ispositioned such that dried waste is discharged from the output end ofthe housing into the furnace, and wherein the supply of hot gases issupplied from the furnace.
 9. The waste incineration system according toclaim 8, further comprising: at least one duct located on an externalsurface of the housing having an opening in the furnace for collectinghot gases and for providing hot gases along the housing to the supply ofhot gases at the input end of the housing; the at least one duct beingpositioned such that hot gases passing through the duct heat a surfaceof the housing.
 10. The waste incineration system according to claim 3,wherein the furnace is a waste-to-energy boiler.